As the size of electronic devices scales down and power densities increase, the demand for innovative cooling solutions becomes more imperative. Thermal interface materials (TIMs) such as thermal greases and gels with highly conductive particle additives are commonly used in microprocessor cooling solutions where operating temperatures are near 100° C. However, recent reliability tests on polymeric TIMs using thermogravitic analysis revealed a dramatic increase in thermal interface resistance as operating temperatures and exposure times increased. Because of their high thermal conductivity, mechanical compliance, and stability over a wide temperature range, carbon nanotubes have been extensively studied as conductive elements. Several recent reports have shown that dense, vertically aligned CNT arrays are viable alternatives to current state-of-the-art TIMs. However, when contact sizes between a nanotube and an opposing surface become comparable to the mean free path of the dominant energy carriers, nanoscale constriction resistance becomes important. For CNT TIMs similar to those in this study, the resistive component at the CNT ‘free tip’ and opposing metal substrate has been shown to cause the largest constriction of heat flow in comparison to the bulk CNT and growth substrate resistances [9]. Reduction of this ‘free tip’ constrictive resistance using novel CNT TIM composite structures is shown in several embodiments of the inventions disclosed herein.
Recent thermal resistance values for CNT based TIMs have been measured to be between 1-20 mm2 K/W. The thermal resistance values include both bonded and non-bonded interfaces, and measurements were obtained using different characterization techniques (1D reference bar, thermoreflectance, photoacoustic, and 3-omega). Weak bonding at heterogeneous interfaces, differences in phonon dispersion and density of states, and wave constriction effects are factors that could hinder further reduction in thermal contact resistance. Adverse phonon constriction can be moderated by increasing the interfacial contact area. In an effort to increase the interfacial contact area, developments in bonded and semi-bonded CNT TIMs have rendered thermal interface resistances as low as 1.3 mm2 K/W and 2 mm2 K/W, respectively. CNTs exhibit ballistic conduction of electrons in the outermost tubes and ohmic current-voltage characteristics with certain metals. When this effect is coupled with a strong metallic-like bond at the CNT/metal substrate interface, phonon constriction could be circumvented by using electrons as a secondary energy carrier. A possible way to achieve electron transmission is through a strong CNT/metal substrate bond and sufficiently high electron DOS at the interface.
Silicon carbide (SiC), with a band gap near 3.3 eV, is attractive for high-temperature power electronic applications such as high-voltage switching for more efficient power distribution and electric vehicles, powerful microwave electronics for radar and cellular communications, and fuel efficient jet aircraft and automobile engines. At high temperatures, phonon scattering with charge carriers increases while the charge carrier mobility, which dictates electrical conductivity, is adversely affected. In order to maintain sufficient electrical conductivities for reliable operation in such applications, innovative heat dissipation methods that can withstand high temperature environments are necessary.
With regards to the performance of a Si/CNT interface with a commercial phase change material (PCM) applied to a CNT array, there is a decrease in thermal resistance of approximately 10 mm2 K/W between Si/PCM/Cu interfaces and Si/CNT/PCM/Cu interfaces, which achieved a low value of approximately 5 mm2 K/W at 350 kPa. Also, there can be a 50% reduction in thermal interface resistance by wicking paraffin wax into CNT arrays grown on both sides of Cu foil. It is possible that such improvement is the result of an increase in contact area and reduction in constriction resistance at the ‘free tip’ interface. However, PCMs and paraffin wax suffer similar disadvantages as polymeric TIMs at high temperatures. In contrast, thermal resistances near 10 mm2 K/W in a dry SiC/CNT/Ag interface can be achieved, with the possibility of a weak dependence of thermal interface resistance on temperatures up to 250° C., indicating that the TIM was suitable for high temperature applications.
What is needed are thermal interface materials and construction methods that have lowered thermal interface resistance and improved long term characteristics. Various embodiments of the present invention do this in novel and unobvious ways.